A base station (eNodeB) in a 3GPP Long Term Evolution (LTE) system periodically transmits synchronization signals and relevant system information. A mobile station (user equipment, UE) shall carry out two essential steps to access the network:
A cell search procedure consisting of a series of synchronization steps upon which the UE acquires time and frequency synchronization and other crucial system parameters that are necessary to demodulate the downlink signals and channels; and
A random access procedure to declare its presence within the cell, obtain the correct uplink timing synchronization and a unique UE identity (C-RNTI).
The cell-search procedure is used in two cases: for initial synchronization, when a UE detects a cell for the first time and decodes the information needed to register to it; and for identifying new cells, upon a UE registered in the network and searches other cells for handover.
The cell-search procedure exploits two specially designed signals: the primary synchronization signal (PSS) and the secondary synchronization signal (SSS). The particular design of these signals allows a UE to acquire the correct time-frequency synchronization (including subframe boundaries), as well as the physical identity of a cell, the cyclic prefix length, and whether the cell operates in time division duplex (TDD) or frequency division duplex (FDD). With this information, a UE can correctly decode the system information broadcasted by the cell in the physical broadcast channel (PBCH), comprising a set of random-access preamble sequences and the corresponding time-frequency resources, known as the physical random-access channel (PRACH), that can be used to access the network.
Random-access is used in LTE for a number of purposes: for initial access to establish a radio link; to re-establish a radio link after a radio link failure; for handover; and for scheduling request when a dedicated resource has not been granted for scheduling purposes. A common aspect to these purposes is the acquisition of the exact uplink timing and to assign a unique identity, the C-RNTI, to the UE. A UE can perform random-access only on its primary component carrier, in either a contention-based or contention-free manner (the latter only used to re-establish uplink synchronization, handover, and positioning). Contention-based random-access consists of four steps:
Transmission, from the UE to the eNodeB, of a random-access preamble which allows the eNodeB to estimate the uplink timing of the UE.
Transmission, from the eNodeB to the UE, of a timing advance response to adjust the terminal timing estimate obtained at the first step.
Transmission of the UE identity (which depends on whether the UE was already known to the network) using UL-SCH similar to scheduled data.
Transmission of a contention resolution message from the eNodeB to the UE using DL-SCH to resolve any contention due to multiple UEs trying to access the cell using the same random-access procedure.
The contention-free random-access consists only of the first two steps. The first step is the only one requiring physical layer processing. The transmission of a random-access preamble is intended to signal a base station a random-access attempt. A set of 64 preamble sequences are available in each cell divided into two subsets. The sequences forming each subset are signaled to the UE through the PBCH. A UE selects at random a sequence in one subset depending on the amount of data to be transmitted in the third step of the procedure. Therefore, an eNodeB cannot detect a random-access attempt to another eNodeB as it utilizes different time-frequency radio resources and different random-access preamble sequences.
Future generations of radio cellular networks may allow base stations to be switched on/off dynamically in order, for instance, to reduce inter-cell interference and increase spectral efficiency, to adapt to traffic changes, or to save energy. In some cases, a base station could be adapted to transit to an intermediate state, a sleeping/dormant mode, in which only part of its functionalities are switched off or operate in a low-energy mode. For instance, the base station of a cell could overhear the uplink activity in neighboring cells while being inactive in the downlink.
The absence of synchronization signals, broadcast channel and any downlink pilot would render a cell “invisible” to any mobile station within its proximity. Dynamic on/off switching of cells has been proven to be particularly beneficial in terms of both energy and spectral efficiency in heterogeneous cellular networks, i.e., radio cellular networks consisting of macro-cell providing radio coverage to a large area and smaller cells (e.g., pico-cell, femto-cells, etc.) offering radio coverage to a smaller area. This new functionality however requires redesigning several aspects of the traditional radio cellular systems.
In a wireless mobile communication system, where access points can dynamically transit from an active state (wherein all downlink and uplink functionalities are fully operational) to an inactive state (wherein at least some downlink functionalities are fully or partially turned off while the uplink functionalities may or may not be turned on) and vice versa, new methods are needed to enable a mobile station to find an access point to connect to or the access point with the best radio link. Further a dual problem is how a network can control, optimize and adapt the access point configuration and status in response to short-term and/or or long-term changes in the radio environment, traffic statistics, user type of traffic and data rate requirements, user mobility and migration, etc.
Dynamic on/off switching of cells in heterogeneous radio cellular networks has been advocated as a mean to increase energy and spectral efficiency. Standardization bodies, such as the 3rd Generation Partnership Project (3GPP), have showed interested in this technology by considering it a potential technique for interference mitigation in deployments of small cells. An enabling technique is the introduction of a new operational state for small cell nodes, a sleep/dormant mode, in which a radio network access node does not transmit any downlink signal (e.g., downlink synchronization signals, broadcast channel, downlink pilots and data) but may still receive uplink signals.
A prior art solution proposes three methods to control the transition of a cell in a radio cellular network from a sleep state to an active state:                Cell self-controlled sleep-mode: assumes the presence of sufficient underlay macro coverage and a low-power sniffer available at the sleeping node. The sleeping node utilizes the power sniffer to detect a rise in the received power when a call is initialized by a UE with a macro-cell. If the received signal strength exceeds a predefined threshold, the UE is deemed close enough to be potentially covered by the sleeping cell and a handover procedure is started.        Core-network controlled sleep-mode: The transition of cell from sleeping to a ready state is controlled by the backhaul using a wake-up message. Also in this case, it is assumed that a UE is initially correctly connected to a macro-cell layer so that the proper core-network element can verify whether there is a sleeping cell, within the macro-cell region, that can serve the UE.        A UE-controlled sleep-mode: places the sleep mode control at the UE side. A UE should broadcasts wake-up signal in order to wake-up sleeping cells within its range. Any time a sleeping cell receives a sensing signal from a UE, the cell would transit to a ready state and serve the UE.        
These methods however have practical shortcomings that prevent them to be efficiently utilized in practical radio cellular deployment. For instance, by detecting a raise in received power, a cell cannot distinguish between a surge of inter-cell interference and a nearby mobile station starting a communication with another cell. In addition, a mobile station needs downlink synchronization and other information to decode new downlink pilots from a dormant base station. The overall handover would be long and cumbersome. The core-network controlled sleep-mode, on the other hand, requires the knowledge of the exact geographical location of all small cell nodes and the relative position of mobile stations, which is unrealistic in practical deployments.
An alternative prior art solution proposes to control the activation of dormant cells by reusing the random-access procedure. This procedure however requires that the mobile station has first synchronized to the downlink timing of the cell and received the RACH resources (time-frequency location and preamble sequences set) for the dormant cell. This, however cannot be accomplished when a cell is inactive in downlink. Therefore, new methods and procedures are needed to allow a mobile station to access the network through an inactive cell (dormant or fully switched off) for which no system information is available at the mobile station.